Disk Utility seems to only be able to partition drives that are physically connected to the computer. I believe some people have been able to create a disk image on the Time Capsule's HDD by mounting it prior to starting Disk Utility, and then creating disk images the size of the partitions they want to use. I'm not exactly sure of how to do this as I haven't had the need. However, if you want to create an actual partition, you need to pop open the TC and seat the HDD into an external enclosure. Then you proceed to partition it with Disk Utility. Again, I'm not sure exactly how to do this, but a search may help you further. Also, opening the TC will void your warranty.
Best thing to do is to use the TC's HDD for backup and connect an external for AirDisk.